The present invention relates to a patterning process for the wiring layer of semiconductor devices, and more particularly to a dry etching process for a thin film aluminum or aluminum alloy.
A thin film of aluminum or aluminum alloy (hereinafter both terms will be represented by aluminum) is generally used for the wiring layer of semiconductor devices including ICs (Integrated Circuits) and LSI (Large Scale Integration) circuits.
Modern manufacturing of ICs of LSIs, employs dry etching, and especially dry etching using reactive gas species. For example, reactive plasma etching or reactive ion etching have become indispensable in fabricating circuit lines and islands having dimensions of a micron or submicron. In structures such as these, side etching is required to be less than a few tenths of a micron for a layer having a thickness of around 1 micron.
In dry etching of an aluminum film, a gas containing chlorine species is usually employed as the etchant because of its higher etching rate for aluminum compared to other etchant gases containing different reactive species, for example, fluorine. FIG. 1 is a schematic cross-sectional diagram illustrating the etching mechanism of an aluminum film by active chlorine atoms (chlorine radicals; Cl*). In the figure, an aluminum film 1 is formed on an insulating layer 2 which is formed on a semiconductor substrate (not shown). The aluminum film is selectively masked by a resist mask film 3, such as ordinary photoresist. At the unmasked portion of the aluminum film, incident ions containing chlorine species (Cl+, CCl.sub.3.sup.+, etc.) and/or chlorine radicals (Cl*) react with the aluminum film 1 to form volatile aluminum compounds, such as aluminum chloride (AlCl.sub.x, where x is considered to be 1, 2 or 3). Since such aluminum chloride compounds are easily removed from the surface of the aluminum film 1 by the thermal agitation or by the bombardment of the incident ions, etching at a higher speed can be maintained. At the same time, side etch, i.e., etching in a lateral direction, takes place under the masked portion of the aluminum film 1 (such side etching is also called under-cut) The side etch shown in the drawings is emphasized.
In the conventional dry etching technique, highly delicate and complicated process control is needed to keep the side etch within the required range mentioned above. The factors to be controlled include the pressure and flow rate of the etchant gas and its composition, the input power for generating plasma, etc. These factors must be tuned individually for each etching system. If the etching conditions are not sufficiently maintained, a large amount of side etch occurs. This side etching often extends under the masked portion of the aluminum film, and completely under-cuts the masked portion before the unmasked portion of the layer is etched off. In other words, such a difficulty in the control of the processing conditions for the dry etching process is one of the causes of poor yields and reduced operational reliability of the devices due to the wiring line being thinned by side etching, which causes burnout failure during field operations.
Side etching is considered to result from the adsorption of the chlorine radicals (Cl*) on sidewall surfaces of tne aluminum film. That is, the chlorine radicals (Cl*) are electrically neutral and able to move independently of the electrical field accompanying the radio frequency power used to generate a plasma for the etching. Therefore, the chlorine radicals can easily diffuse in the lateral direction to reach the sidewall and then attack it to etch it away.
Another problem involved in the conventional dry etching of aluminum film is the production of a number of defects such as a vermicular pattern developed on the surface of the aluminum film, as illustrated in FIG. 2. Such a defect occurs when an aluminum layer, which has been dry etched using a gas containing chlorine species is exposed in the air. This defect is considered to be caused by corrosion of the aluminum film by hydrochloric acid. As described before, aluminum chloride is formed during dry etching and adsorbed on the resist mask film 3. Because the aluminum chloride is strongly hygroscopic and forms hydrochloric acid (HCl), as soon as it absorbs the moisture in the air. The hydrochloric acid penetrates the resist mask film 3, and then corrodes the surface of the aluminum film 1 lying under the resist mask film 3. This corrosion does not occur uniformly over the surface so that the vermicular pattern results. This corrosion decreases the effective cross-section of the aluminum wiring line and increases its resistivity. This eventually leads to burnout of the wiring line when an electric current is applied to the wiring line.
Furthermore, when an aluminum-silicon alloy film is used as the wiring layer, silicon accumulates on the surface of the etched portion of the aluminum film. FIG. 3 is a schematic cross-sectional diagram illustrating a residual polysilicon layer 31 formed on the unmasked portion of the aluminum-silicon alloy film 32 during dry etching. In FIG. 3, reference numerals 2 and 3 designate elements corresponding to those in FIG. 1. The residual polysilicon layer 31 causes a short circuit between wiring lines.
The accumulation of polysilicon is due to the different etching rates of aluminum and silicon, and to the chlorine radicals. The etching rate of silicon by the chlorine radical is less than that of aluminum. Therefore, when the polysilicon layer 31 is formed on the surface, the apparent etching rate of the aluminum-silicon alloy film 32 using the chlorine radical decreases. Accordingly, extensive time or higher input power is needed to etch the entire thickness of the aluminum-silicon alloy film 32. This results in increased side etching or under-cut.